The invention relates to hot water heater control systems and more particularly to a means for regulating the temperature at which the thermostat of the heater is set in response to the actual need or demand for hot water. During periods of low use, heat energy is wasted if the temperature of the water in the storage tank remains at a high level. For example, a temperature of 125.degree. is adequate for all ordinary purposes, and a higher temperature does not ordinarily result in a higher temperature of the water actually used, but merely means that the user will mix more cold water with the hot water for showering, washing hands, and dishes in the sink, etc.
Therefore, a system which would actually lower a setting of the thermostat during periods of low use, and raise it during high utilization times would save energy and would not cause any inconvenience to the family using the water, in a residential situation with a home water heater. Such a system was designed by the instant inventor and is the subject of U.S. Pat. Nos. 4,016,402 and 4,166,944. The system set forth in U.S. Pat. No. 4,166,944 utilizes two temperature sensing switches in series, one mounted on the outgoing hot water pipe and the other mounted on the incoming cold water pipe. The temperature of the cold water pipe adjacent the heater reservoir or tank is cold when water is being used because cool, fresh water is being drawn into the tank. Once the flow stops, however, hot water from the tank through convection rises into the incoming water line, heating it up. Therefore, a temperature sensor switch can recognize a period of utilization by sensing the incoming water temperature, which will fall when water is used.
The switch on the outgoing hot water tank senses a condition in which the heat of the water in this tank is already above a certain point, so that despite present demand for the water, there is no need for a "burn."
Although the system described by the above-mentioned patents works well and does save significant energy, further testing and operational experience has revealed areas wherein improvements to the system can be made. For instance, the system as described in those patents under certain circumstances may rotate the thermostat unnecessarily to the high setting in response to brief, limited demand for hot water. Such a rotation might well be appropriate for periods of continuing demand, such as when using laundry equipment or a dishwasher, but are wasteful when only a short demand period is experienced by the system.